Higher order modulation wireless signals such as those used in CDMA (Code Division Multiple Access) or OFDM (Orthogonal Frequency Division Multiplexing) based communication systems have a high Peak to Average signal power Ratio (PAR). The higher peaks require the communication system to operate a Power Amplifier (PA) used to transmit the signal at less than an optimal power level because the higher signal peaks can cause the PA to max-out or saturate. To allow the power amplifier for a communication system to be driven harder and more efficiently, the peak to average ratio of the transmitted signals should be reduced while preserving the other characteristics of the signal such as modulation accuracy and spectral mask requirements.
Digital PAR reduction techniques typically involve injecting noise into the signal to cancel out the time domain signal peaks, thereby reducing the PAR. Traditionally, finite impulse response (FIR) filters are used to spectrally shape the cancellation noise before applying the cancellation noise to the signal. By so shaping the cancellation noise, spectral re-growth of the signal is prevented. The FIR filter should match the instantaneous spectrum of the composite multicarrier signal typical of transmit systems, otherwise mismatch between the time domain profiles of the signal peaks and the cancellation noise reduces the peak cancellation efficiency and will introduce out of spectrum emissions thus violating the emissions mask.
Multi-carrier communication signals requiring dynamic allocation of carrier frequencies, or dynamic scaling of carrier power, require the FIR filter coefficients to also be recomputed and updated on the fly. For dynamic signals like LTE, carrier power can change widely over bursts as short as 1 ms. Optimal PAR reduction can be obtained when the power distribution of the cancellation pulse matches the power distribution of the signal spectrum. In the absence of prior knowledge of the frequency hopping sequence or the power variations, for a communication signal, estimation of the new FIR filter coefficients to match the new carrier frequency allocations becomes a very hardware intensive problem.